Point Defects and Ferromagnetism in GaMnAs Alloys

Rachel S. Goldman

Professor

rsgold@umich.edu

2094 H.H. Dow Building

T: (734) 647-6821

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In many semiconductors, the introduction of impurities at dilute concentrations leads to dramatic changes in the electronic, optical, and magnetic properties. For example, the introduction of a few percent nitrogen into GaAs leads to a band gap reduction of hundreds of meV. Furthermore, the incorporation of a few percent manganese into GaAs enables a combination of semiconducting and ferromagnetic behavior. In dilute GaAsN alloys, the effects of N incorporation mechanisms on the GaAsN bandgap and electron mobility have apparently not been considered. Our studies reveal that substitutional N incorporation leads to the most significant band gap lowering, while simultaneously leading to the highest electron mobilities reported to date. These advances are due in part to our identification of a forbidden-window of growth for GaAsN. We are in the process of developing an understanding of the relative effects of N clusters and interstitials on the electronic states using a combination of transport studies of modulation-doped heterostructures in conjunction with calculations by a colleague in Ireland. Our long-term goals include tailoring heterostructures for high-performance heterojunction bipolar transistors and high-efficiency multi-junction (tandem) solar cells. In the dilute magnetic semiconductor, GaMnAs, we recently reported the Mn-concentration dependence of various point defect concentrations, revealing a substantial concentration of arsenic anti-sites and vacancies, which likely contribute to compensation of free carriers. We are in the process of expanding these studies in order to develop an atomic-level understanding of carrier compensation and its interplay with magnetism in dilute magnetic semiconductors.